Semiconductor integrated circuits are generally manufactured using a lithographic process. Since the minimum processing dimension of lithography depends on the wavelength of light used, it is necessary to shorten the wavelength of the irradiated light in order to improve the integration degree of the integrated circuit. Specifically, the lithographic process is, at present, performed using a light source having a wavelength of 157 nm to 365 nm. An object is to achieve the practical use of the lithography using an extreme ultraviolet light source having a wavelength 11 nm to 14 nm.
As a light source for generating the extreme ultraviolet light, a light source using a laser plasma method has been studied. According to this method, a target is irradiated with a laser beam to form plasma, and extreme ultraviolet light emitted from the plasma is used.
The emission efficiency of the extreme ultraviolet light will be described referring to FIG. 1. Regarding the horizontal axis, the position 0 μm corresponds to the position of a surface of a target (reference numeral 15), the region with a negative value in the horizontal axis (left region from the surface 15) corresponds to the inside of the target and the region with a positive value in the horizontal axis (right region from the surface 15) corresponds to the outside of the target. When the target is irradiated with a laser beam from the right end toward the left side in the graph, that is, in the direction of an arrow 13, the surface of the target is ablated and plasma blows out to the outside of the target. The plasma is in a quasi steady state while the intensity of the laser beam is constant. Distribution curves 18 and 19 show atomic density of elements which form the target and the plasma. The distribution curves 18 and 19 in the region with a negative value in the horizontal axis show the atomic density (initial density) of the target in the solid state, and those in the region with a positive value in the horizontal axis show the plasma density of the elements. The plasma density exponentially decreases as the distance from the surface 15 is larger.
The energy of the laser beam irradiated to the target is absorbed in a laser absorption region 11. The absorbed energy is, as shown by a reference numeral 14, transported from the laser absorption region 11 to an extreme ultraviolet light emission region 12. The extreme ultraviolet light is emitted in the extreme ultraviolet light emission region 12 owing to the transported energy.
The inventors found that an energy loss occurs during transport of the energy between the two regions and came to the idea that by adjusting the density of the target so as to make the distance between the laser absorption region and the extreme ultraviolet light emission region smaller, the emission efficiency of the extreme ultraviolet light can be improved (Patent document 1). The principle is as follows.
The density of the plasma generated when the target is irradiated with the laser beam depends on the initial density of the target. When the target initial density is high, the plasma exists widely from the surface (distribution curve 18) and when the target initial density is low, the plasma exists only in the vicinity of the surface (distribution curve 19). The laser absorption region 11 is a so-called cut-off electron density region of the plasma, which is defined according to the following equation. That is, with respect to the wavelength λ of the laser beam,c/λ=[(e2ncr)/(∈0me)]1/2  (1)
(where c, e, ∈0, me, ncr are light velocity, unit charge amount, vacuum dielectric constant, electron mass and electron density, respectively). As the target initial density becomes smaller, the cut-off density region moves toward the surface 15 (to a downstream region with respect to the direction of the laser beam irradiation) (arrow 17). On the other hand, the condition by which the plasma emits the extreme ultraviolet light depends on temperature as well as density, and the extreme ultraviolet light emission region 12 is closer to the target surface when the target initial density is low compared to the high target initial density. To make the laser absorption region 11 closer to the extreme ultraviolet light emission region 12, the density of the target should be made small. Thus, in patent document 1, a low-density target is used and the density of the low-density target is 0.5% to 80% of the crystal density of the heavy metal.
However, a plasma having a lower density than the density of the emission region exists in the far side from the surface 15 when viewed from the emission region (an upstream side of the laser beam). The plasma reabsorbs the extreme ultraviolet light and emits light having a longer wavelength than the extreme ultraviolet light. As a result, the emission efficiency of the extreme ultraviolet light is decreased. Thus, in Patent document 1, a low-density target such as a heavy metal (or heavy metal compound) target having a cavity therein or a frost-like heavy element target is used. Accordingly, by making [thickness×density] of the plasma generated on the upstream side of the emission region smaller, reabsorption of the extreme ultraviolet light can be suppressed.
The heavy metals used for the target include Ge (germanium), Zr (zirconium), Mo (molybdenum), Ag (silver), Sn (tin), La (lanthanum), Gd (gadolinium), W (tungsten). Among the metals, Sn has the highest absorption efficiency of laser beam and can emit the extreme ultraviolet light most efficiently. The wavelength of the extreme ultraviolet light obtained from the target using Sn is 13.5 nm. By using Cu or Mo as the heavy metal, an X-ray of a shorter wavelength can be obtained.
[Patent document 1] International Publication No. WO2004/086467 (page 3, line 1 to page 5, line 20, and FIGS. 1 to 3 and FIG. 5)